The components of the classical and lectin pathways have been found in chickens (Barta O & Hubbert NL 1978; Lynch et al 2005). Complement activation results in the proteolytic cleavage of C3 to C3b and C3a, a reaction that is mediated by the C3 convertase. C3 convertase cleaves C3 component to generate a large amount of C3b, that binds to the target surface. The other cleavage product, anaphylatoxin C3a, initiates an inflammatory response. In mammals, in the classical and lectin-mediated pathways the C3 convertase is formed from the surface-bound C4b complexed with C2b (C4b:C2b). In the alternative pathway, factor B serves as the catalytic subunit of C3 convertase. In mammals, factor B and C2 share extensive amino acid homology; they have the same exon and intron organization and are located in tandem on the same chromosome within the mammalian MHC class III region (Carroll MC et al. 1984; Campbell RD & Bentley DR 1986; Cross SJ & Thomson W 1990; Salter-Cid L & Flajnik MF 1995; Nonaka M & Kimura A 2006). For these reasons, the two proteins are thought to have originated by gene duplication from an ancestral molecule. It remains unclear in which animal phyla the duplication event took place.
Chicken factor B-like protease (factor B) was found to be equally related to mammalian complement components B and C2A (Kjalke M. et al. 1993). In addition, a homologue for C2 was not found in chickens and factor B seemed to participate in both classical and alternative pathways of complement activation (Barta O & Hubbert NL 1978; Kjalke M. et al. 1993). It was assumed that the role of C2 may be fulfilled by the chicken factor B-like protease, an evolutionary remnant of a common C2/factor B ancestor (Kjalke M. et al. 1993).